This invention relates to improved methods and apparatus for drilling wells with a top drive drilling unit.
In a top drive drilling system, the motor which turns the drill string is connected to the upper end of the string and moves downwardly with the string during the drilling operation. Equipment of this type has enjoyed increasing popularity in recent years because it usually permits a substantial reduction in the overall cost of drilling as compared with the standard rotary system having a rotary table and kelly. However, for drilling in some areas the rotary table arrangement has had one advantage in that it requires the drill string to be pulled upwardly off of the bottom of the hole each time an added length of pipe is connected to the upper end of the string. This may reduce the possibility of the string becoming stuck in the hole, as may occur when a string remains on the bottom without rotation while adding pipe to its upper end. Also, when drilling an offshore well from a floating vessel, elevation of the string off of the bottom of the hole while adding pipe to the string prevents damage to the string or other equipment or the well by intermittent movement of the lower end of the string into and out of engagement with the bottom of the well as the vessel moves upwardly and downwardly with wave motion.
In top drive systems as previously utilized, it has not in most instances been possible to lift the string off of the bottom of the hole while adding pipe to the upper end of the string. For this reason some drillers have resisted use of top drive units in certain environments, such as for example in formations where there is a tendency to become stuck, or in floating vessel installations.